Ever since the introduction of the rifled gun barrel, various types of bands of comparatively soft material known as obturators have been utilized around the circumference of a projectile for engaging the rifling to a sufficient extent to effect a seal for preventing blowby of gases, and to assure that the projectile will be rotating at a rapid rate as it leaves the gun barrel. The typical rotational rate may be 200 revolutions per second, which serves to provide a desirable amount of stability for an ordinary projectile.
With the advent of ammunition that includes special type warheads, it has become desirable to provide a means for limiting the spin rate of a projectile to a low number of revolutions per second. Along this line the Thompson U.S. Pat. No. 3,208,345 proposed an expander disc arranged to move forward upon the firing of the projectile, with this disk serving to expand the rearwardly extending flange of a disc such that it effectively engages the rifling of the weapon tube. This arrangement was sometimes satisfactory for use with certain projectiles, but it was found to be too heavy and complicated for use with projectiles fired from large bore gun barrels, and in addition, it could not withstand the considerable heat built up by a gun barrel after repeated firings.
It was quite obvious that a very definite need existed for a decoupling obturator which would serve the multiple, often conflictory purposes of providing an effective seal to prevent the undesirable escape of gases on the one hand, while on the other hand effectively decoupling the projectile so that it would spin only at a rate of say 5 to 20 revolutions per second as it left a rifled gun barrel, which is roughly 1/10th the spin rate that would ensue if a suitable decoupling means were not provided.